I could have chosen to talk on appealing, fashionable topics, like Dirac, Majorana and Weyl fermions. But the true ones, as rigorously established in elementary particle physics [1], and not as completely misunderstood in condensed matter [2]. I could also have chosen to bore you all talking on my recent, epochal discovery [3]. Mor e modestly, this will be an informal seminar in the hope to stimulate the interest on ab initio theories of experimental groups in the department/laboratory, suggest possible applications of these theories in their research programs, in the hope of collaborations with us. I will first present my own interests, essentially towards the development of both many-body density-functional and Green function theories to describe electronic excitations and spectroscopy. Then, taking examples from my present and past research activities and collaborations with the experiment, I will present my expertises. I will try to convince you about the high accuracy achieved by ab initio theories on the calculation of dielectric properties and energy-loss spectra in technological materials, like high-k HfO2 studied [4] at LETI in HREELS TEMs (in view of a TEM upgrade at NEEL). Or on the calculation of excitations, the momentum distribution and the dynamic structure factor S(q,w) in simple metals [5] and semiconductors [6], up to more complex HTSC (YBCO) [7], like measured at ESRF (trying a parallel with the S(q,w) of He4 and 3 at ILL). Or also applications to topics like the electronic structure and charge-density waves in transition-metal dichalcogenides [8] or other strongly-or-weakly correlated materials of interest for experimental groups at MCBT. Upon request I could present the ab initio possibilities on nano- and molecular electronics quantum transport [9]. Last and not least, I will conclude with our most recent work on the excitations in that one which has always to be considered as "The" system : Helium [10].